1. Field of the Invention
The present invention relates to an electronic endoscope system in which an endoscope image is reproduced as a full color image on a TV monitor, and, in particular, to such an electronic endoscope system with a simulated dye-spraying process or color-balance alteration process, which is constituted such that the endoscope image can be reproduced on the TV monitor as if it were sprayed with a dye-solution.
2. Description of the Related Art
As is well known, an electronic endoscope system includes a video scope, inserted in an organ of a human body, having a solid-state image sensor for capturing an organ image or endoscope image as a frame of image-pixel signals, an image-signal processing unit for producing a video signal based on the frames of image-pixel signals successively read from the solid-state image sensor, and a TV monitor for reproducing the endoscope image as a motion picture based on the video signal fed from the image-signal processing unit.
Recently, it is usual to manufacture an electronic endoscope system such that the endoscope image is reproduced as a full color motion image on a TV monitor. Thus, a dye-spraying examination method was developed and has been used as a medical examination method in the medical field in which electronic endoscope systems are used. For example, when a subtle uneven surface of the mucous membrane of a stomach or a colon is examined, the dye-spraying medical method is utilized.
In particular, the mucous membrane surface of the stomach or the colon features a reddish orange tone as a whole, and thus it is very difficult to examine the subtle unevenness of the mucous membrane surface. In order that the subtle unevenness of the mucous membrane surface can be clearly and easily examined on a TV monitor, a bluish solution, such as an Indigo Carmine solution, is introduced into a forceps-insertion passage of the video scope, and is sprayed over the mucous membrane surface. The solution has a tendency toward gathering at fine recess areas on the mucous membrane surface, and it flows away from fine land areas on the mucous membrane surface. Namely, the fine recess areas on the mucous membrane surface are colored blue, and clearly contrast with the reddish orange areas. Thus, it is possible to easily carry out an examination of the subtle unevenness of the mucous membrane surface.
However, there are various drawbacks in the dye-spraying medical examination method. For example, a dye must be harmless to a human body, and it is troublesome to develop a harmless dye. Also, an introduction of a dye-spraying medical examination method prolongs the medical examination time when using the electronic endoscope system, resulting in an increase in the patient's pain. Further, once a dye-solution is sprayed, it is impossible to immediately reproduce an endoscope image without the sprayed dye-solution.
In order to settle the above-mentioned problems, Japanese Laid-Open Patent Publication (KOKAI) No. 2001-25025 discloses an electronic endoscope system with a simulated dye-spraying process for electronically processing an endoscope image as if it were sprayed with a blue-solution.
In this electronic endoscope system, a full color endoscope image is formed based on a frame of three-primary color image-pixel signals, which is composed of a frame of red image-pixel signals, a frame of green image-pixel signals, and a frame of blue image-pixel signals. In the simulated dye-spraying process, for example, a value of a central red image-pixel signal is compared with an average of values of eight circumferential red image-pixel signals surrounding the central red image-pixel signals.
If the value of the central signal is lower than the average of the values of the circumferential signals, the central red image-pixel signal derives from a fine recess area on a mucous membrane surface of, for example, a stomach. However, if the value of the central signal is higher than the average of the values of the circumferential signals, the central red image-pixel signal derives from a fine land area on the mucous membrane surface of the stomach. The same is true for the green image-pixel signals and the blue image-pixel signals.
Accordingly, for example, if the frame of three-primary color image-pixel signals is processed such that the values of red and green image-pixel signals, deriving from the fine recess areas, are lowered, an endoscope image can be reproduced as if it were sprayed with a bluish-solution.
Further, before the simulated dye-spraying process can be properly performed, it is necessary to take account of a spatial frequency of an endoscope image captured by the image sensor. Nevertheless, the aforesaid KOKAI No. 2001-25025 does not refer to the spatial frequency of the endoscope image captured by the image sensor.
In particular, when an unevenness on the mucous membrane surface of the stomach is captured by the image sensor, the captured unevenness image exhibits a specific spatial frequency. On the other hand, an image sensor has a specific pixel pitch, which is defined as an array pitch of photodiodes arranged on the light-receiving surface of the image sensor. In this case, for example, when the spatial frequency of the captured unevenness image is too low in comparison with the pixel pitch of the image sensor, it is impossible to properly perform the simulated dye-spraying process, because the eight circumferential image-pixels, surrounding the central image-pixel, do not necessarily represent a land area surrounding the fine recess represented by the central image-pixel signal.
For example, when an endoscope image to be reproduced on the TV monitor is enlarged, the spatial frequency of the enlarged endoscope image becomes lower than that of the original endoscope image, resulting in degraded performance of the simulated dye-spraying process. Also, when the video scope is substituted for another type of video scope featuring a solid-state image sensor having a smaller pixel pitch, the simulated dye-spraying process cannot be properly performed, because the spatial frequency of the endoscope image captured by the other type of video scope becomes relatively lower, due to the smaller pixel pitch of the image sensor thereof.